Constant wattage heating element

ABSTRACT

An electrical resistance heating element having a constant wattage per unit length comprises a conductor rod enclosed by a braided tubular insulator which in turn is enclosed by a braided tubular conductor. Strands of yarn having a high electrical conductivity and a high electrical resistance are interwoven between the inner and outer surfaces of the tubular insulator to form a plurality of parallel resistors between the conductor rod and tube.

United States Patent [1 1 Weckstein 1 CONSTANT WATTAGE HEATING ELEMENT[75] Inventor: Raymond A. Weckstein,

Tulsa, Okla.

[73] Assignee: Sola Basic Industries, Inc.,

Milwaukee, Wis.

[22] Filed: June 15, 1973 [21] Appl. No.: 370,414

[52] US. Cl 219/552, 174/110, 219/529,

219/545, 219/549, 338/214, 338/224 [51] Int. Cl. 1105b 3/10 [58] Fieldof Search 219/528, 529, 545, 549,

[56] References Cited UNlTED STATES PATENTS 2,264,285 12/1941 Bennett338/214 X 2,322,773 6/1943 Peters 338/224 X Challemer 338/214 Buchananet a1. 338/214 X 1 Jan. 7, 1975 2,905,919 9/1959 Lorch et a1. 338/2243,309,643 3/1967 Ferretti et a1, 338/330 3,349,225 10/1967 Dubois219/545 3,398,233 8/1968 De Lizasoain et a1. 174/110 3,454,746 7/1969Dubois 219/549 3,501,619 3/1970 Buiting-et a1. 219/553 X 3,757,0869/1973 lndoe 219/528 Primary Examiner-Volodymyr Y. Mayewsky Attorney,Agent, or FirmSmythe & Moore 1 1 ABSTRACT An electrical resistanceheating element having a constant wattage per unit length comprises aconductor rod enclosed by a braided tubular insulator which in turn isenclosed by a braided tubular conductor. Strands of yarn having a highelectrical conductivity and a high electrical resistance are interwovenbetween the inner and outer surfaces of the tubular insulator to form aplurality of parallel resistors between the conductor rod and tube.

3 Claims, 3 Drawing Figures Patented Jan. 7 1975 -CONDUCT|NG YARN 0NOUTER DIAMETER 0F BRAID F-IG.3

heater cables generally comprise a pair of conductors,

separated from each other by a semi-electrically conducting material ora material which is electrically conductive but has a high electricalresistance. The conventional heater cable constructed in the foregoingmanner constitutes a series-connected resistance device with a totalelectrical resistance of the cable depending upon its length. Suchheater cables have the disadvantage that they must be cut to apredetermined length depending upon their proposed use, and this lengthcannot be varied since to do so would vary the electrical resistancecharacteristics.

It has been proposed to construct an electrical heater cable having aconstant wattage which would produce a given watt density or watts perunit length for a given voltage independent of the length of the cable.While several forms of such electrical heater cables have been proposed,they have had the disadvantage that the manufacturing thereof wascomplicated and expensive. Further, it has been difficult to constructsuch a cable having a desired electrical resistance per unit length forvarious lengths of the cable.

One of the objects of the present invention is to provide an improvedelectrical heater cable which has a constant wattage per unit length.

Another of the objects of the present invention is to provide such anelectrical heater cable which is relatively inexpensive to manufactureand simple in construction.

According to one aspect of the present invention, an electricalresistance heating element may comprise a conductor rod enclosed by atube of electrically insulating material. A tubular conductor enclosesthe tubular insulator. Means are provided in the tubular insulator todefine a plurality of parallel discrete-like resistors between theconductor rod and tube so that the heating element has a constantwattage per unit length. The insulating tube may comprise a braid ofinsulating yarn within which are interwoven strands of a yarn having ahigh electrical conductivity and a high electrical resistance. Thesestrands are woven between the outer and inner surfaces of the tubularinsulator braid so as to form electrical conductors between theconductor rod and tube.

Other objects, advantages, and features of the present invention will beapparent from the accompanying description and drawings which are merelyexemplary.

In the drawings:

FIG. 1 is a perspective view of a portion of an electrical heater cableaccording to the present invention with successive layers of the cablebeing cut away to show the details of construction.

FIG. 2 is a perspective diagrammatic view of the tubular insulatingbraid showing the manner in which a strand of electrically conductivematerial is interwoven therethrough; and,

FIG. 3 is a longitudinal sectional view diagrammatically showing theconducting yarn in the tubular insulator braid.

Proceeding next to the drawings wherein like reference symbols indicatethe same parts throughout the various views, a specific embodiment ofthe present invention will be described in detail.

As may be seen in FIG. 1, there is indicated generally at 10 anelectrical heater cable according to the present invention. The cablecomprises a copper or suitable conductor rod or wire 11 surrounded by acoaxial tubular braid l2 woven from an electrically insulating yarn. Oneor more strands of an electrically conductive yarn 13 which has a highelectrical resistance are interwoven in the insulating braid 12 so thatportions of the strand 13 are exposed on the inner surface 14 and outersurface 15 of the tubular braid In this manner, a number of electricallyconductive high resistance paths are formed through the insulator.

One example of electrically conductive strands is a strand of fiberglass or quartz which can be subjected to millimicron-size particles ofa highly conductive material in a colloidal suspension. Materials suchas graphite, silicon carbide and other semiconducting materials can beused. One colloidal suspension is Aquadag" E, Aquadag being a trademarkof Acheson Colloids Co. for colloidal graphite in water with pHcorrection additives and other compounds that enhance adhesion to glass.Merely as an example, the material could have a resistance of 8 megohmsper foot.

The insulating braid 12 is in turn surrounded by a coaxially positionedbraid or conductor means 16 of electrically conductive material such ascopper strands.

Surrounding the conductor braid 16 is a protective sheath formed of anelectrically insulating material which in turn may be enclosed by anouter protective sheath of stainless steel or suitable material. Theouter casing 17 could also comprise a stainless steel foil havingstainless steel wire braid or seam welded stainless steel sheath.

The result of interposing a braid of insulating material havingelectrically conductive strands interwoven therein between a rodconductor and a tubular conductor is to form in effect a pair ofconductors with a plurality of resistors connected between them inparallel. The result is a heater cable which a user can cut to anydesired length in the field, and the resultant cut cable will maintainits given unit wattage or resistance.

It is to be understood that changes in various details of constructionand arrangement of parts may be made without departing from the spiritof the invention ex- 7 cept as defined in the appended claims.

What is claimed is:

1. An electrical resistance heating cable comprising an electricalconductor rod, a coaxial tubular electrical insulating braid enclosingsaid conductor, a tubular electrically conductive braid enclosing saidcoaxial tubular insulating braid, and strands of electrically conductiveyarn of high electrical resistance in said coaxial tubular braid exposedat spaced points on the inner and outer surfaces of said coaxial tubularbraid defining a plurality of high resistance paths between saidconductor rod and coaxial tubular braid whereby the heating element hasa constant wattage per unit length.

2. An electrical resistance heating element as claimed in claim 1wherein said coaxial tubular insulating braid comprises a braid ofinsulating yarn.

3. An electrical resistance heating element as claimed in claim 2wherein strands of a yarn having a high electrical conductivity and ahigh electrical resistance are interwoven in said braid of insulatingyarn.

1. An electrical resistance heating cable comprising an electricalconductor rod, a coaxial tubular electrical insulating braid enclosingsaid conductor, a tubular electrically conductive braid enclosing saidcoaxial tubular insulating braid, and strands of electrically conductiveyarn of high electrical resistance in said coaxial tubular braid exposedat spaced points on the inner and outer surfaces of said coaxial tubularbraid defining a plurality of high resistance paths between saidconductor rod and coaxial tubular braid whereby the heating element hasa constant wattage per unit length.
 2. An electrical resistance heatingelement as claimed in claim 1 wherein said coaxial tubular insulatingbraid comprises a braid of insulating yarn.
 3. An electrical resistanceheating element as claimed in claim 2 wherein stranDs of a yarn having ahigh electrical conductivity and a high electrical resistance areinterwoven in said braid of insulating yarn.